Bleaching a mixture of sulfite and semi-chemical pulps



United States Patent BLEACHING A MIXTURE OF SULFITE AND SEMI-CHEMICAL PULPS Lawrence W. Murtfeldt, Wisconsin Rapids, Wis., assignor to Consolidated Water Power & Paper o., Wisconsin Rapids, Wis., a corporation of Wisconsin No Drawing. Application March 31, 1951, Serial No. 218,695

6 Claims. (Cl. 8-105) This invention relates to chemical refining and bleaching of a mixture of digested cellulose pulps wherein one of the pulps has a lignin content and a bleach consumption or demand several times as great as the other, such as for example, a mixture of a conventional sulfite pulp and a ligno-cellulose semi-chemical pulp prepared by the neutral sodium or ammonium sulfite process.

Although mixed pulps, such as groundwood and sulfite 2,723,179 Patented Nov. 8, 1955 The reason for such prior separate handling of each pulp resulted from the belief that it would not be feasible to bleach them in admixture due to their great difference in chlorine demand, particularly in the first chlorination step, it having been considered that the joint bleaching of such dissimilar pulps would result in over-bleaching of and cellulose damage to the low chlorine demand sulfite pulp, and under or over-bleaching of the higher chlorine demand semi-chemical pulp.

I have, however, in accordance with the presentinvention unexpectedly found that two pulps of the diverse character and greatly different lignin content previously described, may be jointly bleached without detriment, and with unexpected advantages. Thus, more specifically, I have found that if I prepare a mix of, for example one to four parts by weight of a sulfite pulp prepared by the calcium bisulfite process together with one to four parts by weight of a semi-chemical pulp prepared by the neutral sodium sulfite process and add to this mix, in the initial or chlorination stage of the aforesaid three-stage bleaching process, a total amount of chlorine equal to that which is required by or which normally would or should have been added to the aforesaid pulps if treated separately, that unexpectedly each of said pulps takes up and reacts with the amount of chlorine it requires. That is to say, the chlorination in this stage unexpectedly proceeds as if the two pulps were being handled separately, without underchlorine to chlorinate the lignins of the pulp, then subjected to alkaline solubilizing agent, such followed by final bleaching with an oxidizing agent, such as a hypochlorite, to desired brightness such as 80-85 G. E. meter brightness, with washing following each step.

' Hemlock, spruce, balsam and the like woods digested I by the sulfite process, contain about 1 /2-2 lignin and can readily be bleached by the indicated three-stage process to desired brightness to produce white grades of book and the like papers. The pulp is of a relatively free nature and can be readily handled and particularly washed and tent of from about 6% to about 12% and is more difficult to bleach, although it may also be bleached in the indicated three-stage process. However, the pulp has a low degree of fineness, drains very slowly and requires over-size washers for the same capacity as sulfite. It has a chlorine total consumption of from about 15% to about 20% and requires from about 8% to about 15 chlorine in the first stage to bring it to an 80-85 G. E. brightness for use such as book paper.

It is often desirable to prepare a mixture of sulfite and semi-chemical pulps to provide a mixed pulp which has a composite of the respective physical properties of the individual pulps, and this has heretofore been accomplished by separately bleaching each pulp and subsequently mixing them. Such procedure has obvious disadvantages in that the semi-chemical pulp per se is relatively difficult to process, as previously indicated, and either requires alternate bleaching of separate batches of the individual pulps, or requires provision of separate installations for the bleaching of individual pulps.

or over-bleaching or cellulose damage to the sulfite pulp or over or under bleaching of the semi-chemical pulp. Moreover, I find that this first or chlorination stage tends to equalize the chlorination demands of such dissimilar pulps so that requirements in the second and third stages are substantially the same as when bleaching sulfite pulp alone.

Although a semi-chemical pulp when bleached alone 'drains very slowly in the requisite inter-stage washing steps, and requires oversize washers for the same capacity as sulfite pulps, I have found in accordance with the present process that the washing rate is not appreciably slowed up in ratios of up to about 2 of sulphite to 1 of semichemical pulp. This, of course is an important economical advantage, and enables the use of a conventional three-stage sulfite bleach plant, and avoids the necessity for providing a separate one for handling this semi-chemical pulp. I

I have found that by proceeding in accordance with the mixed bleaching process of the present invention, that, for example, a mixture of pulps of the aforesaid character treated in the three-stage bleaching process results in a pulp having the same mullen, tensile and tear strengths as does a mixture prepared from the same two pulps when sep arately bleached.

As previously indicated, the three-stage bleaching process herein contemplated, when applied to either of the aforesaid pulps alone, employs from about 50% to about 70% of thetotal chlorine consumption or as expressed herein, chlorine demand in the initial stage as chlorine water. The same holds true when treating a mixture of pulps in accordance with the present invention. That is to say, I add to the mixture of pulps of characteristically divergent chlorinedemand or consumption, a total amount of chlorine in the first stage equivalent to 50 to 70% of the chlorine demand of the respective pulps calculated separately. Thechlorine demand or consumption of a sulfite pulp may be readily determined by the indicated TAPPI Test T214M-42. The chlorine demand of semichemical pulp cannot be readily determined by said test but is ascertainable through actual use. I have found however, that an amount of chlorine roughly equivalent to the lignin content or say approximately 10% of the dry weight of the pulp is adequate in the first stage, and

has been found adequate in the indicated three-stage process, whether the semi-chemical pulp is bleached alone or in admixture as per the present invention. The lignin content of the semi-chemical pulp may be readily determined by TAPPI Test T222M. Thus, expressed in another Way, I add to a mixture of sulfite pulp and semichemical pulp, in the first chlorination stage of the bleaching process an amount of chlorine equal to 50 to 70% of the chlorine demand of the sulfite pulp as determined by TAPPI Test T214M-42, plus approximately by weight of the unbleached oven dry semi-chemical pulp and which is roughly equivalent to the lignin content thereof which is readily determined by TAPPI Test T222M.

Treatment in this first stage is otherwise conventional, that is, the slurry is well agitated until the chlorine is practically exhausted, after which it is washed in its acid state on a vacuum washer. It is then subjected to a second or alkaline treatment stage to substantially neutralize the pulp and to solubilize and remove the chlorine reaction products of the first stage, after which the pulp in slightly alkaline condition is again washed. It is then subjected to a final bleaching or oxidizing step by treatment with a hypochlorite to attain final desired G. E. brightness, the amount of hypochlorite being here determined by the chlorine demand of the mixed batch in accordance with TAPPI Test 214M-42, in conventional amount, followed by final washing.

Although I have hereinbefore indicated that I may employ up to about 1 part of semi-chemical pulp to 2 parts by weight of sulfite pulp without appreciably slowing up the washing rate of the pulp mixture as compared to the use of a sulfite pulp alone and thus obtain an important economical advantage, it will be understood that lower proportions of the semi-chemical pulp may be used and likewise higher proportions thereof may be employed, although in the latter instance with somewhat lesser although still appreciable advantage.

The following is an illustrative example of a conventional method of bleaching a standard sulfite pulp in a three-stage process:

Example 1.-Sulfite pulp, cooked to a 6% to 8% chlorine demand, expressed according to TAPPI Standard Test T214M42, and having a lignin content of 1% to 2% is treated at 3 /2 to 4% consistency with a 2% to 3% chlorine water in the amount of 50% to 70% of the total chlorine required to bleach to an 80 General Electric meter brightness. Chlorination proceeds with the pulp slurry well agitated until the chlorine added is practically exhausted. Temperatures in the chlorination stage are seasonal, varying from 40 F. in winter to 75 F. in the summer. The pH of the pulp after chlorination is 1 /2 to 2.

The chlorinated pulp is washed in its acid state on a vacuum washer, being discharged at 11 to 14% consistency to caustic bleachers. Sodium hydroxide at concentration is added during filling of the bleacher in the amount of 1% to 2% based on the weight of unbleached oven dry pulp.

The temperature of the pulp is raised to 120 to 140 F. in this stage. Time in the stage is one hour from the end of filling to dumping. pH of the pulp after caustic and pulp are mixed is 7.5 to 8.0. The pulp is then washed with fresh water over a vacuum washer and discharged at 12% to 14% to final stage bleachers. Lime is added between washer and bleacher sufficient to attain a pH of 8.0 to 8.5 at the completion of bleaching.

Calcium or sodium hypochlorite in 30 gram per liter concentration is added to the pulp in the amount of to 2% based upon chlorine demand as expressed by TAPPI Test T214M42. Temperature in this stage is 90 to 95 F. After practical exhaustion of chlorine, the bleach pulp is dumped and washed with fresh water.

The following is an illustrated example of a conventional method for bleaching a semi-chemical pulp in a three-stage process:

Example ll.--A hardwood semi-chemical pulp made by cooking with neutral sodium sulfite or neutral ammonium sulfite followed by mechanical refining, and containing 8% to 11% of lignin as expressed by TAPPI Test T222M, is bleached in the same plant as heretofore described but with the following chemical use differences:

Chlorine as chlorine water is added in the first stage in the amount of 10% to 11% based on the weight of unbleached oven dry pulp.

Two per cent of sodium hydroxide is used in the second stage, and 1% to 2% of chlorine as calcium or sodium hypochlorite in the final stage. Temperature, pH, and consistency is substantially the same in all stages as with sulfite bleaching described above.

The following is an illustrative example of a method of bleaching of mixed pulps in accordance with the present invention:

Example [IL-Neutral semi-chemical pulp and sulfite pulp are thoroughly mixed at 3% consistency in pro portions in the range of 20% by weight of semi-chemical and by weight of sulfite to 80% of semi-chemical and 20% of sulfite.

This procedure is as follows:-The unbleached sulfite is tested for chlorine demand by TAPPI Test T214M-42, and 50% of the chlorine requirement is added in the chlorination stage based on the pounds of oven dry sulfite in the batch. 10% of chlorine based on the pounds of oven dry semi-chemical pulp in the batch is added in the chlorination stage.

For example, assume a mixture of 75% sulfite and 25 semi-chemical, .the sulfite testing 7.0% chlorine demand. First stage chlorine added per ton of unbleached mixed pulps will be:

75% of 2000 lbs.=1500 lbs.

25% of 2000 lbs.=500 lbs.

1500 7.0 50%=52.5 lbs. chlorine 500 10%=50.0 lbs. chlorine 52.5+50.0=102.5 lbs. chlorine added per ton of unbleached mixed .pulp in the chlorination stage.

The chlorinated pulp is then processed exactly the same as individual pulps heretofore described, using 1% to 2% of sodium hydroxide in the second stage, and 1% to 2% of chlorine as calcium or sodium hypochlorite in the third stage, based on TAPPI Test T2l4M-42 for chlorine demand ahead of the third stage.

The resulting bleached pulp contains 23.6% of bleached semi-chemical and 76.4% of bleached sulfite. This final proportion differs from the unbleached proportion described above due to the difference in weight loss in bleaching of the two pulps, sulfite loss being 8% .and semi-chemical 15%.

The mixed bleached pulp has been proved to be fully equal in all physical qualities, mullen, tear, and tensile,

and density to an identically proportioned and intimately Per- Per- Canadian cent cent Standard Bulk Mullen Tear Freeness (1) Sulfite bleached separately 49 1. 66 655 1.56 (2) Semi-chemical bleached separately. 102 1.24 1.47 (3) Mixture of (1) and (2) after bleaching. Proportions-62% sulfite, 38% semi-chemical I. 74 1. 32 605 1. -18 (4) Mixed Bleached-fi2% sulflte, 38% Semi-chemical bleached together. (Originally made up of 60% sulfite and 40% semi-chemical) 81 1. 40 620 l. 44

I claim'as my invention: 1. The method of bleaching a mixture of digested cellulose pulps composed of sulfite pulp and semi-chemical pulp, said pulps having a characteristically divergent chlorine demand, in a multiple stage bleaching process wherein the pulps are treated in a Water slurry with chlorine in a first stage, followed by treatment with an alkaline reagent and with a hypochlorite in a final stage to reach desired brightness, the step which comprises ing agent to desired brightness, with washing following each step, the improvement which comprises adding chlorine to said first stage in a total amount equivalent to from about 50% to about 70% of the chlorine demand of the respective pulps contained in said mixture.

3. The method of bleaching a mixture composed of sulfite pulp and semi-chemical pulp in a three-stage bleaching process Wherein'the pulps are treated in a water slurry with chlorine to chlorinate the lignins thereof in the first stage, then subjected to alkaline solubilizing treatment, and finally to bleaching with an oxidizing agent to desired brightness, with washing following each step, the

improvement which comprises adding chlorine to said first stage in a total amount equivalent to from about 50% to about 70% of the chlorine demand of the sulfite pulp and about of the dry weight of the semi-chemical pulp.

4. The method of bleaching a mixture composed of sulfite pulp and semi-chemical pulp in a three-stage bleaching process wherein the pulps are treated in a Water slurry with chlorine to chlorinate the lignins thereof in the first stage, then subjected to alkaline solubilizing treat- 4 ment, and finally to bleaching with an oxidizing agent to desired brightness, with washing following each step, the improvement which comprises adding chlorine to said first stage in a total amount equivalent to from about 50% to about 70% of the chlorine demand of the sulfite pulp and an amount approximately equivalent to the weight of the lignin content of the semi-chemical pulp.

5. The method of bleaching a mixture composed of sulfite pulp and semi-chemical pulp in a three-stage bleaching process wherein the pulps are treated in a water slurry with chlorine to chlorinate the lignins thereof in the first stage, then subjected to alkaline solubilizing treatment, and finally to bleaching with an oxidizing agent to desired brightness, with Washing following each step,

the improvement which comprises adding chlorine to said first stage in a total amount equivalent to from about to about of the chlorine demand of the sulfite pulp and an amount approximately equivalent to the weight of the lignin content of the semi-chemical pulp, and adding hypochlorite to said final stage in an amount equivalent to the chlorine demand of the mixture at said stage to provide desired final brightness.

6. A method for facilitating the removal of high lignin content and for bleaching a semi-chemical digested cellulose pulp in a process Where pulp is treated in a water slurry with chlorine in a first stage to chlorinate the lignins thereof, then subjected to alkaline solubilizing treatment and finally to bleaching with an oxidizing agent in a final stage to desired brightness, with washing following each stage, the improvement which comprises initially mixing up to about one part by weight of said semi-chemical pulp with two parts by weight of a relatively low lignin content sulfite pulp, said pulps having characteristically divergent chlorine demands in said process, and simultaneously subjecting said pulps in admixture in the first stage of said process with chlorine in a total amount equivalent to from about 50% to about 70% of the chlorine demand of the sulfite pulp plus an amount approximately equivalent to the weight of the lignin content of the semi-chemical pulp.

References Cited in the file of this patent UNITED STATES PATENTS 1,106,994 DeVains Aug. 11, 1914 1,768,820 Bradley July 1, 1930 2,127,765 Casciani Aug. 23, 1938 2,510,595 McEwen June 6, 1950 OTHER REFERENCES Lee: Bleaching GroundWood-Sulphite Pulp, Chemical Engineering, Oct. 1947, pp. 92-96, -143.

Kauifniann: Hydrogen Peroxide in the Pulp and Paper industry, Paper Trade Journal, Nov. 8, 1945, pp. 33-38.

ODonoghue: Pulp Bleach Plant Design Factors," Paper Trade Journal, Sept. 26, 1940, pp. 41-46.

Moffitt: Objectives in the Various Stages of Pulp Purification, Reprint from the June, 1939, issue of Pacific Pulp & Paper industry, 2 pp.

Casciani and Storin: A Study of pH Factor in Hypochlorite Bleaching of Wood Pulp, Part II-Two-Stage Treatment of [Chlorinated] Sulphite Wood Pulp, Paper Trade 1., Oct. 1, 1942, pp. 89-97.

Sprout: Comparative Studies in the Chlorination of Wood-Pulp, Paper Trade 1., Mar. 13, 1947, p. 45.

Coster: Some Aspect of the Chlorination of Sulphite Pulp, Technical Assn. Papers, Series XXIV, 1941, pp. 204-208. 

1. THE METHOD OF BLEACHING A MIXTURE OF DIGESTED CELLULOSE PULPS COMPOSED OF SULFITE PULP AND SEMI-CHEMICAL PULP, SAID PULPS HAVING A CHARACTERISTICALLY DIVERGENT CHLORINE DEMAND, IN A MULTIPLE STAGE BLEACHING PROCESS WHEREIN THE PULPS ARE TREATED IN A WATER SLURRY WITH CHLORINE IN A FIRST STAGE, FOLLOWED BY TREATMENT WITH AN ALKALINE REAGENT AND WITH A HYPOCHLORITE IN A FINAL STAGE TO REACH DESIRED BRIGHTNESS, THE STEP WHICH COMPRISES ADDING CHLORINE TO SAID FIRST STAGE IN A TOTAL AMOUNT EQUIVALENT TO FROM ABOUT 50% TO ABOUT 70% OF THE CHLORINE DEMANDS OF THE RESPECTIVE PULPS CONTAINED IN SAID MIXTURE. 